1. Field of the Invention
This invention is related in general to electrolytic processes and equipment for refining copper. In particular, it describes a method of construction for an improved electrolytic cathode.
2. Description of the Related Art
The principle of electrolysis has been utilized for decades to extract metals and other cations from an electrolytic solution. The extraction process is carried out by passing an electric current through an electrolyte solution of the metal of interest, such as copper, gold, silver, or lead. The metal is extracted by electrical deposition as a result of current flow between a large number of anode and cathode plates immersed in cells of a dedicated extraction tank house. The anode is made of a material that is dissolved and, therefore, is lost during the process, while the cathode is generally constructed of a metal alloy, such as titanium or copper alloys and various grades of stainless steel (316L, 2205, etc.), resistant to corrosive acid solutions. In the most efficient processes, each cathode consists of a thin sheet of metal of uniform thickness (2-4 mm) disposed vertically between parallel sheets of anodic material, so that an even current density is present throughout the surface of the cathode. A solution of metal-rich electrolyte and various other chemicals, as required to maintain an optimal rate of deposition, are circulated through the extraction cells; thus, as an electrical current is passed through the anodes, electrolyte and cathodes, a pure layer of electrolyte metal is electro-deposited on the cathode surface, which becomes plated by the process.
Similarly, to purify a metal in a refinery process using electro-deposition, an anode of impure metal is placed in an electrolytic solution of the same metal and subjected to an electric current passing through the anode, electrolyte and cathode of each cell. The anode goes into solution and the impurities drop to the bottom of the tank. The dissolved metal then follows the current flow and is deposited in pure form on the cathode, which typically consists of a starter sheet of stainless steel. When a certain amount of pure metal has been plated onto the starter sheet, the cathode is pulled out of the tank and stripped of the pure metal.
In both processes, the pure metal deposit is grown to a specific thickness on the cathode during a predetermined length of time and then the cathode is removed from the cell. It is important that the layer of metal deposited be recovered in uniform shapes and thicknesses and that its grade be of the highest quality, so that it will adhere to the cathode blank during deposition and be easily removed by automated stripping equipment afterwards. The overall economy of the production process depends in part on the ability to mechanically strip the cathodes of the metal deposits at high throughputs and speeds without utilizing manual or physical intervention. To that end, the cathode blanks must have a surface finish that is resistant to the corrosive solution of the tank house and must be strong enough to withstand their continuous handling by automated machines without pitting or marking. Any degradation of the blank's finish causes the electro-deposited metal to bond with the cathode resulting in difficulty of removal and/or contamination of the deposited metal.
Also immensely important in the production and refining of metals by electrolytic extraction is the relationship of electrical power consumption with metal-production rates. The total weight of deposited metal can be calculated theoretically by knowing the actual energy used, the concentration of metal in solution, the average residence time, the number of cells, and the surface area available for deposition in each cell. In practice, all electrical voltages and flow rates are continuously monitored throughout the deposition cycle to optimize the electrolytic process. After the cathodes have been pulled out of the cells and the deposited metal has been stripped and weighed, the electrolytic-product weight is divided by the theoretical cell-production weight to determine the cell efficiently. A cell efficiency of ninety-five percent or better is the goal for the best operations.
In order to achieve this level of efficiency, the voltage profile across the cathodic deposition surface must be held constant and variations avoided. Shorts due to areas of high current density caused by nodulization or by curved cathode surfaces that touch the anode must be prevented. Therefore, the details of construction of cathode blanks are very important to minimize operational problems and ensure high yields.
U.S. Pat. No. 4,186,674 to Perry (1980) describes a cathode for the electrolytic refining of copper that has been considered as the state of the art in the industry. It consists of a stainless steel hanger bar point-welded to a stainless-steel starter sheet hanging from it in vertical position (see FIG. 1). The hanger bar has a flat bottom face for maximum surface contact and corresponding maximum electrical conductance with support bus-bars through which the system is energized. In order to reduce the electrical resistance resulting from the welds between the hanger bar and the starter sheet, the hanger bar and the upper edge of the starter sheet are uniformly clad with copper, thereby creating a low-resistance boundary between the two. In addition, in order to prevent deposit build-up along the lateral edges of the starter sheet that would impede the automated separation of the product at the end of each cycle, these edges are masked with an insulating strip riveted to the electrode.
While amounting to a substantial improvement over the prior art, the Perry cathode retains some features that have proven to cause problems from time to time. The flat bottom face of the hanger bar tends to remain positioned in full contact with the bus-bars even when the starter sheet is not perfectly perpendicular to it because of warpage or construction defects. The result is that the starter sheet does not hang perfectly vertical and its distance from the surrounding anodes is not uniform, sometimes being even in shorted contact thereto. This condition causes nonuniform deposits that affect the efficiency of operation and the quality of the product.
Another problem arises when, due to wear, pits and faults develop in the copper cladding around the hanger bar. Then the steel underneath (the material of which the bar is made) is exposed to the corrosive atmosphere of the electrolytic tank house, rapidly leading to a build-up of high-resistance corrosion spots that decrease the conductivity of the whole electrode. Such corrosion eventually causes enough structural damage to require replacement of the hanger bar and reconditioning of the cathode. In addition, after the copper plating is sufficiently worn out to become ineffective as a conductor at the boundary between the hanger bar and the starter sheet, the current flow is restricted to the points welded between them, which have a relatively high resistance and therefore affect the efficiency of the cathode as well.
Finally, the method shown by Perry for securing the insulating strips to the lateral edges of the cathode is rather cumbersome and requires chemical bonding to avoid bulging between pin fasteners. Therefore, it is not suitable for rapid replacement of damaged strips.
In view of the above, there still exists a need for an improved electrolytic cathode that overcomes these problems. The present invention provides a simple method of construction for producing electrodes that fulfill this need.